1. Field of the Invention
This invention relates to a wireless telephone system, and more particularly, is applicable to a digital codeless-phone system in which plural portable units are registered on a base unit to communicate between the base unit and a portable unit or between the portable units.
2. Description of the Related Art
Heretofore, as a digital codeless-phone system of this kind, there is a system of, for instance, a personal handyphone system (PHS) scheme wherein wireless communication is performed between the base unit, which has been connected to a public telephone network by wire, and the portable units or among the portable units, by the use of the time division multiple access (TDMA) scheme. In this digital codeless-phone system of PHS-scheme, intercommunication may be performed among the plural portable units which have been registered on the base unit or between the base unit and the portable units, and communication may be performed between a portable unit and a telephone which has been connected to the public telephone network, so-called line wire.
On the TDMA scheme in the digital codeless-phone system of PHS-scheme, the time base is divided into 5 millisecond [ms] frames. The respective frames are divided into eight time slots in amount, that is, the continuous four transmission time slots and the continuous four reception time slots. The duration of each time slot is 625 millisecond [.mu.s].
By using one transmission time-slot in one frame on the base-unit side and using one reception time-slot on the portable unit side, the PHS-scheme digital codeless-phone system transmits the digital data which is comprised of the physical slots of the stated bit number from the base unit to the portable unit through a certain frequency. Besides, by using one transmission time-slot in one frame on the portable unit side and using one reception time-slot on the base-unit side, the PHS-scheme digital codeless-phone system transmits the digital data which is comprised of the physical slots of the stated bit number from the portable unit to the base unit through the same frequency as the abovementioned frequency.
The PHS-scheme digital codeless-phone system causes the portable units which are in a reception waiting state to intermittently receive only the control data transmitted from the base unit, so that the power dissipation of the portable units of this state is decreased. That is, the base unit transmits the control data to the respective portable units, using 625 [.mu.s] for each 5 [ms], that is, using one time slot for each two-hundred time slots. At this time, the base unit transmits the control data intermittently by the use of a dedicated frequency (hereinafter, this is referred to as a control carrier).
On the other hand, the respective portable units which are in the reception waiting state receive only the control data which has been transmitted using the control carrier, once for each stated period, for instance, once for every one or two seconds.
When a request to connect has occurred, the base unit transmits the individual number of a called-side portable unit by means of the control data, so as to call up the portable unit which has this individual number within one or two seconds. After the portable unit has been called up, the base unit discriminates the contents of the connection request, such as a calling or reception to the line wire, an intercommunication with the base unit, and an intercommunication with the other portable unit, and then assigns a time slot for transmission/reception of the voice data, and an arbitrary frequency (hereinafter, this is referred to as a communication carrier) excepting the frequency for a direct talk between portable units and that portable unit.
By this, the respective portable units which are in a reception waiting state are not needed to operate in a mode of regular searching of, for instance, ten communication carriers for a direct talk between portable units, and to check whether a call for intercommunication between portable units exists or not. Therefore, the respective portable units receive only the control data which has been transmitted from the base unit through the control carrier, so as to decrease considerably the power dissipation of the time of a reception waiting state in comparison with that of continuous reception, and communicate with the base unit or communicate with a portable unit via the base unit.
Here, as shown in FIGS. 1A to 1C, an explanation is given of such a state that, after the base unit CS which accommodates two time slots has transmitted the control data through the control carrier and assigned the transmitting/receiving time slots and the communication carrier of the frequency ft, the base unit is relaying the speech between the portable units PS1 and PS2, where T1 to T4 are transmission time slots and R1 to R4 are reception time slots. This base unit which accommodates two time slots comprises only one oscillator for selecting frequency (radio channel) in its radio section, and switches the oscillating frequency of this, so as to generate the communication carrier of the arbitrary frequency.
As shown in FIG. 1B, the portable unit PS1 transmits a digital voice data to the base unit CS by the use of the communication carrier of the frequency ft, at the transmission time slot T1 of the period S1. As shown in FIG. 1A, the base unit CS receives this voice data at the reception time slot R1 of the period S1, and then transfers this voice data to the portable unit PS2 at the transmission time slot T3 of the period S7, by the use of the communication carrier of the frequency ft. As shown in FIG. 1C, the portable unit PS2 receives this voice data at the reception time slot R1 of the period S7, and causes the time slot to be synchronized.
On the other hand, the portable unit PS2 transmits a voice data to the base unit CS by the use of the communication carrier of the frequency ft, at the transmission time slot T1 of the period S3. As shown in FIG. 1A, the base unit CS receives this voice data at the reception time slot R3 of the period S3, and then transfers this voice data to the portable unit PS1 at the transmission time slot T1 of the period S5, by the use of the communication carrier of the frequency ft. As shown in FIG. 1B, the portable unit PS1 receives this voice data at the reception time slot R1 of the period S5, and causes the time slot to be synchronized.
In this manner, the portable units PS1 and PS2 perform the mutual delivering and receiving of the respective voice data via the base unit, and cause the time slots to be synchronized, so that a talk between the portable units is performed.
However, in a case that the digital code-less phone system has the other portable unit PS3 in addition to the portable units PS1 and PS2, there has been such a problem that the base unit CS is not able to transmit the control data to the portable unit PS3 by the use of the control carrier whose frequency is different from that of the communication carrier ft, before the talking between the portable unit PS1 and the portable unit PS2 is terminated.
As a result, such a trouble has occurred that the portable unit PS3 which are in a reception waiting state are not able to receive the control data which is utilizing the control carrier from the base unit CS, during the talk between the portable units PS1 and PS2, and they are put out of the speakable zone. Besides, even if a connection request which is based on an outside-line communication, an intercommunication, a door-phone, etc. has occurred, the base unit CS has been not able to inform the portable unit PS3 which is in a reception waiting state that the connection request has occurred.
In order to solve this, it may be considered to transmit the control data by the use of the control carrier during the talk between the portable units, using a base unit CS which accommodates three time slots. That is, as shown in FIG. 2B, the portable unit PS1 transmits a voice data to the base unit CS by the use of the communication carrier of the frequency ft, at the transmission time slot T1 of the period S12. As shown in FIG. 2A, the base unit CS receives this voice data at the reception time slot R2 of the period S12, and then transfers this voice data to the portable unit PS2 by the use of the communication carrier of the frequency ft at the transmission time slot T4 of the period S18. As shown in FIG. 2C, the portable unit PS2 receives this voice data at the reception time slot R1 of the period S18, and causes the time slot to be synchronized.
On the other hand, the portable unit PS2 transmits a voice data to the base unit CS by the use of the communication carrier of the frequency ft, at the transmission time slot T1 of the period S14. As shown in FIG. 2A, the base unit CS receives this voice data at the reception time slot R4 of the period S14, and then transfers this voice data to the portable unit PS1 by the use of the communication carrier of the frequency ft, at the transmission time slot T2 of the period S16. As shown in FIG. 2B, the portable unit PS1 receives this voice data at the reception time slot R1 of the period S16.
As shown in FIG. 2A, in addition to relaying of the voice data of the portable units PS1 and PS2, the base unit CS transmits a control data an a down control-data to the portable unit PS3 by the use of the communication carrier of the frequency fc, at the transmission time slot T1 of the period S15. Besides, at the reception time slot R1 of the period S11, the base unit CS receives an up control-data which has been transmitted from the other portable unit toward the base unit by the use of the control carrier of the frequency fc.
In this connection, the control data is transmitted and received once for each 5 [ms] (25 frames), as stated above.
In this manner, the base unit becomes capable of transmitting/receiving the control data to/from the portable unit PS3, as well as relaying the voice data of the portable units PS1 and PS2, owing to the use of 3 time slots. But, in this state, such pairs of time-slots are produced so that they are neighboring and their frequencies differ from each other, such as the reception time slots R1 and R2, the reception time slot R4 and the transmission time slot T1, and the transmission time slot T1 and the transmission time slot T2.
However, when it is tried to obtain the frequencies fc and ft by switching the oscillation frequency of a single oscillator for frequency selecting, it is hard to ensure the waiting time for stabilization of the frequency after switching. Therefore, as shown in FIG. 3, it has been the only way that a radio section 1 of the abovementioned conventional base unit CS which accommodates 3 time slots is provided with two frequency-selecting oscillators 2 and 3, and these two oscillators 2 and 3 are switched alternately by a VCO switch 4 so as to obtain the frequencies fc and ft. Therefore, such a problem has existed that the constitution of the radio section of the base unit CS is complicated.
In this connection, the frequency-selecting oscillators 2 and 3 have been composed into frequency synthesizers, each of which has a phase locked loop (PLL) 6 and a voltage controlled oscillator (VCO) 5.